Convert admin api docs to markdown (#10089)

So that they render nicely in mdbook (see #10086), and so that we no longer have a mix of structured text languages in our documentation (excluding files outside of `docs/`).

docs/admin_api/register_api.md

@@ -0,0 +1,73 @@
+# Shared-Secret Registration
+
+This API allows for the creation of users in an administrative and
+non-interactive way. This is generally used for bootstrapping a Synapse
+instance with administrator accounts.
+
+To authenticate yourself to the server, you will need both the shared secret
+(`registration_shared_secret` in the homeserver configuration), and a
+one-time nonce. If the registration shared secret is not configured, this API
+is not enabled.
+
+To fetch the nonce, you need to request one from the API:
+
+```
+> GET /_synapse/admin/v1/register
+
+< {"nonce": "thisisanonce"}
+```
+
+Once you have the nonce, you can make a `POST` to the same URL with a JSON
+body containing the nonce, username, password, whether they are an admin
+(optional, False by default), and a HMAC digest of the content. Also you can
+set the displayname (optional, `username` by default).
+
+As an example:
+
+```
+> POST /_synapse/admin/v1/register
+> {
+   "nonce": "thisisanonce",
+   "username": "pepper_roni",
+   "displayname": "Pepper Roni",
+   "password": "pizza",
+   "admin": true,
+   "mac": "mac_digest_here"
+  }
+
+< {
+   "access_token": "token_here",
+   "user_id": "@pepper_roni:localhost",
+   "home_server": "test",
+   "device_id": "device_id_here"
+  }
+```
+
+The MAC is the hex digest output of the HMAC-SHA1 algorithm, with the key being
+the shared secret and the content being the nonce, user, password, either the
+string "admin" or "notadmin", and optionally the user_type
+each separated by NULs. For an example of generation in Python:
+
+```python
+import hmac, hashlib
+
+def generate_mac(nonce, user, password, admin=False, user_type=None):
+
+    mac = hmac.new(
+      key=shared_secret,
+      digestmod=hashlib.sha1,
+    )
+
+    mac.update(nonce.encode('utf8'))
+    mac.update(b"\x00")
+    mac.update(user.encode('utf8'))
+    mac.update(b"\x00")
+    mac.update(password.encode('utf8'))
+    mac.update(b"\x00")
+    mac.update(b"admin" if admin else b"notadmin")
+    if user_type:
+        mac.update(b"\x00")
+        mac.update(user_type.encode('utf8'))
+
+    return mac.hexdigest()
+```